Superantigens are extremely potent T cell stimulators produced by a variety of microbial pathogens. When complexed with Class II products of the major histocompatibility complex (MHC), superantigens stimulate T cells in a novel way, predominantly via the T cell receptor (TCR) Vbeta element. Previous studies indicate that residues on the solvent-exposed lateral surface of TCR Vbeta8.2 are critical for interaction with the mouse mammary tumor virus (MTV)-encoded superantigen, Mls-1a. It is not yet clear whether these findings can be generalized to all TCR/superantigen interactions. Indeed preliminary data suggest that they may not be. During the proposed granting period, TCR Vbeta elements will be genetically manipulated to determine the TCR structural characteristics required for recognition of two model superantigens: an MTV-encoded Vbeta3-specific superantigen, and the exogenous superantigen of Mycoplasma arthritidis. The role of other TCR variable elements is stabilizing the TCR/superantigen/MHC trimolecular complex will also be addressed. As murine thymocytes mature in the presence of endogenous superantigens, the maintenance of self tolerance necessitates clonal elimination of superantigen-reactive thymocytes. This massive Vbeta-specific deletion dramatically influences the T cell repertoire. Preliminary data indicate that mice expressing structurally identical endogenous superantigens eliminate thymocytes at distinct stages of maturation. The distribution of such superantigens will be determined, with the aim of elucidating whether differences in the distribution of the superantigens result in the differential elimination of T cells. The proposed in situ analyses might indicate the precise physical sites at which T cell recognition of viral superantigens alters the T cell repertoire. Knowledge gained during these studies will be crucial to our understanding of the mechanisms of tolerance induction. The long range goal of these studies is a molecular dissection of the TCR interaction with superantigen/MHC class II complexes during T cell development.